When the weather is cold but not cold enough for a jacket.
Its a method to work out if its to cold outside.
No jacket no problem, stand outside for 5 minutes if you get hard nipples it's cold and you need a jacket.
Is it cold outside? Yes it's hard nipple weather!
Do I need a jacket? Well it's definitely hard nipple weather!
Cold enough to make your nipples hard but not cold enough for a jacket.
Its cold outside will I need a jacket?
Well it's definitely hard nipple weather!